Filters are commonly used in many different applications to ensure that a liquid meets a certain standard of purity or cleanliness. In one example, fluids such as gasoline and hydraulic fluid is filtered to ensure that there are no particles in the fluid that might damage an engine or a pump. In another example, gas such as an exhaust is filtered to minimize the pollution generated by an engine. The applications in which such filters are used are endless and include automobiles, tractors, farm equipment, construction equipment, and machinery.
In a typical application, fluid flows through a filter, which removes foreign matter or particles from the fluid. These filtered particles accumulate in the filter element. As these particles accumulate, the filter element becomes plugged or clogged and loses its effectiveness. As result, the amount of foreign matter in the fluid that escapes through the filter will increase to a dangerous level and may damage the equipment that uses the fluid or may allow an unacceptable level of contaminates to escape into the atmosphere.
In the past, one structure that has been used to monitor the effectiveness of the filter is a pressure differential switch. Such a switch monitors the fluid pressure on both sides of the filter element. As the filter element becomes plugged, the pressure differential across the filter increases. Accordingly, the pressure differential switch is calibrated to close when the pressure differential rises above a predetermined level. The switch can then cause some event to occur such as activating a warning alarm or a warning light. One example of such a pressure differential switch is disclosed in U.S. Pat. No. 4,480,160, which is entitled Differential Pressure Switch and issued on Oct. 30, 1984, the disclosure of which is hereby incorporated by reference.
One difficulty with these preexisting pressure differential switches is that they typically have only a single output. They are either opened or closed, and their state or output changes only when the pressure differential across the filter element crosses single predetermined threshold value. As a result, the preexisting pressure switches can provide only limited information. For example, they cannot provide both a warning signal that indicates a filter element is at the lower limits of acceptable performance and also a warning signal that indicates when the filter has failed. Thus the switch must be set to either provide an operator with a warning signal that the filter is reaching is failure point or a warning that the filter has actually failed. In the first scenario, the operator does not have any warning that the filter has failed. In the second scenario, the operator does not have warning the filter is reaching its limit of acceptable performance and thus does not have any warning to perform preventative maintenance until after the filter actually fails and exposes equipment to damage.
Another difficulty is that the output or warning signal of typical pressure differential switches for filters is mechanical. The switch merely provides a visual indicator for an operator when it is tripped. The filter does not provide electronic accumulation of information, which can be used for a variety of useful purposes. For example, such information would enable a computer to control operation of the equipment utilizing the filter and prevent damage if a filter fails. In another example, such information could be used to monitor filter maintenance for warranty purposes.